1. Field of the Invention
The present invention relates to internal combustion engines, in particular to an exhaust arrangement for a reciprocating or rotary internal combustion engine.
2. Related Art
There is a continuing need to improve the efficiency and fuel economy of modern internal combustion engines. One area of inefficiency relates to the ‘breathing’ of the engine. During the ‘breathing’ of the engine the piston does work to draw the fresh air or air-fuel mixture into the cylinder and then expel it after the compression and power strokes. This breathing takes work (known as pumping losses) directly from the output of the engine and, hence, is detrimental to the fuel economy of the engine (more notably on throttled engines). Much effort is being put into operating the engine with minimal pumping losses to improve fuel economy.
To operate more efficiently modern engines may be ‘boosted’ by an inlet compressor which may be driven by the engine (supercharging) or by an exhaust turbine (turbocharging). This reduces inlet pumping losses and also increases power allowing the engine to be downsized (reduced swept volume). An engine driven compressor however undesirably takes work from the engine. Using a turbine in the exhaust to drive the inlet compressor in a turbocharging system extracts waste energy from the exhaust and avoids taking work from the engine further improving efficiency. More specifically after combustion and the main power stroke, and prior to the exhaust valve opening, cylinder pressures are typically higher than exhaust system pressures. This results in an initial flow of gas out of the cylinder as the exhaust valve is opened, referred to as ‘blowdown’, which can drive the turbine, although subsequently the piston must do work to expel the remaining exhaust gases against the exhaust system pressure. However with all the exhaust gas flowing through the exhaust turbine there is an increased pressure drop across the turbine during the exhaust stroke. This pressure acts on the piston increasing pumping losses on the exhaust, and so offsetting some of the benefit.
It has been proposed to provide a divided exhaust arrangement to reduce the exhaust pumping losses associated with such turbine arrangements. Such arrangements have been proposed as long ago as 1921 in GB 179926, and more recently in GB 2185286, U.S. Pat. No. 6,883,319, and in a paper titled “Divided Exhaust Period—A Gas Exchange System for Turbocharged SI Engines” by C. E. Möller, P. Johansson, B. Grandin and F. Lindström, (SAE Technical Paper 2005-01-1150, 2005). In such proposals the exhaust is split and a suitable valve arrangement directs the exhaust to the turbine during the first ‘blowdown’ exhaust period to drive the inlet compressor, and then during a second exhaust period, bypasses the turbine to thereby reduce exhaust back pressure and allowing the exhaust to more easily vent from the engine so reducing exhaust pumping losses. While proving an improvement over conventional turbocharging arrangements, there are still significant exhaust pumping losses associated with the bypass flow, and these may even be increased, as compared to a non turbocharged exhaust, due to the loss of energy and gas inertia from the initial exhaust blowdown used by the turbine.
Another, to some degree opposite proposal to that of turbocharging, is to place a pump or compressor in the exhaust to positively extract the exhaust gases from the engine and thereby reducing any pumping losses and work done by the piston in expelling the exhaust gases, or even in the extreme to provide a positive load on the piston. Examples of such proposals are described in WO 9728360 in which the exhaust pump is driven by the engine, similar to a supercharging arrangement, and in U.S. Pat. No. 4,439,983 in which the exhaust compressor is driven by an inlet turbine driven by the inflow of fresh air or air-fuel mixture into the cylinder, somewhat similarly to a reverse turbocharging arrangement. In both of these proposals however while exhaust pumping losses are reduced, work is either taken from the engine, or the inlet pumping losses are increased.
Accordingly while all of these arrangements reduce pumping losses and/or improve efficiency, these gains are to some degree off set and reduced by other losses. For example in the case of a turbocharger arrangement to increase engine power and improve engine efficiency, there is an increase in exhaust pressure and pumping losses. In the case of an exhaust pump driven by an inlet turbine replacing the conventional throttle there is a loss of engine responsiveness. In a supercharger, or an exhaust pump, that is driven by the engine some engine power is used with associated performance and efficiency loss. These proposals therefore represent compromises, as well as having other problems. Indeed only turbocharging arrangements have been widely adopted.
It is therefore desirable to provide an improved internal combustion engine exhaust arrangement, and indeed internal combustion engine arrangement, which addresses the above described problems and/or which more generally offers improvements or an alternative to existing arrangements. More specifically there is a need for an improved internal combustion engine exhaust arrangement which reduces pumping losses in particular exhaust pumping losses and/or which improves efficiency of exhaust and/or of the engine overall.